Development of High-Specificity Fluorescent Probes to Enable Cannabinoid Type 2 Receptor Studies in Living Cells

Pharmacological modulation of cannabinoid type 2 receptor (CB₂R) holds promise for the treatment of numerous conditions, including inflammatory diseases, autoimmune disorders, pain, and cancer. Despite the significance of this receptor, researchers lack reliable tools to address questions concerning the expression and complex mechanism of CB₂R signaling, especially in cell-type and tissue-dependent contexts. Herein, we report for the first time a versatile ligand platform for the modular design of a collection of highly specific CB₂R fluorescent probes, used successfully across applications, species, and cell types. These include flow cytometry of endogenously expressing cells, real-time confocal microscopy of mouse splenocytes and human macrophages, as well as FRET-based kinetic and equilibrium binding assays. High CB₂R specificity was demonstrated by competition experiments in living cells expressing CB₂R at native levels. The probes were effectively applied to FACS analysis of microglial cells derived from a mouse model relevant to Alzheimer's disease.

SAT-598 Shared Signaling Profile Between Human MRAPα-Induced Human MC4R Constitutive Activity and Obesity-Associated Human MC4R Constitutive Activity

The human melanocortin 4 receptor (hMC4R) plays a critical role in the regulation of energy balance with more than 150 distinct human obesity-associated mutations. Most exhibit defective MC4R functionality but six have been reported to associate with constitutive activity. This represents a conundrum since a lean phenotype is expected for enhanced MC4R signaling. Human melanocortin 2 receptor accessory protein alpha (hMRAPα) induces hMC4R constitutive activity in transfected HEK293 cells (1,2). We do not know whether the hMRAPα-induced gain-in-function for hMC4R would cause, or prevent, obesity because of this conundrum. Here, we hypothesize that wild-type hMC4R, obesity-associated constitutively active hMC4R and hMRAPα-induced constitutive active hMC4R can exist in distinct conformational states and elicit distinct signaling profiles. To test this, we compared transiently expressed HA-hMC4R in HEK293 cells for basal and agonist activation for adenylyl cyclase, Cre driven β-galactosidase reporter transcription, and receptor protein expression. Six previously reported obesity-associated hMC4R constitutively active variants were compared with two hMC4R constitutively active mutations not associated with obesity, two hMC4R variants associated with protection from development of obesity, five non-constitutively active hMC4R mutations associated with obesity, hMRAPα co-expressed with hMC4R, and wild-type hMC4R. Our data confirm hMC4R constitutive activity coupling to both adenylyl cyclase and Cre β-galactosidase reporter for only two hMC4R variants associated with obesity (H76R & L250Q), one hMC4R mutation (H158R) not associated with obesity, and hMRAPα co-expressed with hMC4R. We show α-MSH stimulated concentration curves for wild-type hMC4R, H76R, L250Q & H158R hMC4R variants and hMRAPα co-expressed with hMC4R coupling to adenylyl cyclase. Surprisingly, out of these, only wild-type hMC4R and H158R hMC4R variant exhibited α-MSH-stimulated Cre β-galactosidase reporter concentration curves. Western blotting and ELISA showed ~70% reduced cell surface and total receptor protein expression for hMC4R co-expressed with hMRAPα and obesity-associated constitutively active hMC4R variants, compared to wild-type hMC4R. To summarize, two constitutively active hMC4R variants (H76R and L250Q) associated with obesity, and hMC4R co-expressed with hMRAPα, share a signaling profile comprising protein expression and α-MSH stimulated functional coupling to adenylyl cyclase and Cre-reporter gene expression. We conclude (1) if hMC4R is co-expressed with hMRAPα in vivo it would likely contribute to human obesity, and (2) obesity-associated constitutively active hMC4R variants exhibit a signaling anomaly that may underpin development of anti-obesity therapeutics.

1. Kay EI, et al. J Mol Endocrinol. 2013;50:203-215.

2. Kay EI, et al. PLoS ONE. 2015;10(10):e0140320.